For a long time, shoe press devices, i.e. presses having an extended nip, have been employed when manufacturing different paper and paperboard grades, primarily for wet-pressing in order to increase the dryness of the web, but also for calendering and other finishing in order to improve surface properties or other physical properties of the web.
As a result of their longer nips, shoe press devices provide a number of advantages in comparison to conventional roll presses, such as higher dryness at the same nip pressure, or a pressing at a lower nip pressure but with maintained dryness which is more gentle to the sheet.
In order to regulate the press nip conditions in a shoe press, such as the pressure curve in the machine direction throughout the nip, it is often desirable to be able to displace the pressure shoe forward or backward in the machine direction, to thereby affect the pressure curve through the press nip in such a way that the press power is at its highest in the beginning and lower towards the end of the press nip, or vice versa. At such regulation the pressure shoe will, in principle, follow an arc-shaped line of movement centred in the central axis of the counter roll.
The reason for the desirability of a regulated pressure curve in the machine direction throughout the press nip is, among others, that different qualities of paper or paperboard have different optimal pressure curves.
U.S. Pat. No. 4,713,147 describes a shoe press with an extended press nip in a paper machine, in order to remove water from a wet fibrous and porous paper or paperboard web. The press comprises a rotating press roll and at least one loading shoe, which is pressed towards the press roll to form a prolonged pressing zone. The loading shoe is capable of rotating around an axis parallel to the axis of the press roll. The web to be dewatered is passed through the pressing zone together with at least one felt that receives water from the web. The loading shoe is supported on a carrier frame by supporting means. The center of gravity of the supporting force exerted by the supporting means at the shoe is displaceable in the dewatering direction of the web in order to allow adjustment of the pressure pattern generated in the pressing zone when different paper qualities are processed. The displacement of the center of gravity is performed either mechanically, by displacing the supporting means in relation to the shoe, or hydraulically, by varying the pressures in two separate pressure cushions in the supporting means.
Further, SE-C2-510 609 describes a shoe press for a paper or paperboard machine comprising a pressing shoe and a counter roll, between which is formed an extended nip for a paper or paperboard web, and a circulated flexible belt. The shoe press further comprises at least one press cylinder arranged between a horizontal beam and the press shoe, in order to press the latter against the counter roll. The press shoe is arranged on top of the press cylinder, whereby there is a device for anchoring the press cylinder to the horizontal beam, which can be disconnected. After the disconnection of the anchoring device, the press cylinder may be affected by eccentric members for displacement in the machine direction, and for the regulation of the pressure profile of the shoe press.
In the publication EP 0 933 471, corresponding to U.S. Pat. No. 6,083,352, the disclosure of which is incorporated herein by reference, a shoe press is disclosed which reduces the problems originating from the thermal expansion of the press shoe, since it has the ability to tolerate relatively large elongations of the press shoe across the machine direction, and also other deformations of the press shoe. The disclosed shoe press comprises a press shoe extending in the cross-direction along the entire width of a web running through the press, and a plurality of articulated hydraulic loading cylinders supported by a support and spaced apart along the shoe. The loading cylinders define working chambers which can be pressurised by hydraulic fluid, so as to enable the cylinders to force the press shoe away from the support and towards a counter roll or other backing member for applying pressure to the web being carried through the nip defined between the shoe and the backing member. Each loading cylinder comprises a piston member disposed within a cylinder member. Either the piston or the cylinder comprises a two-part member having a first member fixed relative the press shoe and a second member fixed relative to the support, while the remaining piston or cylinder comprises a coupler. Since the press shoe of the shoe press disclosed in EP 0 933 471 also can be moved or tilted in the machine direction in relation to the support, the shoe press comprises a stop member restricting the movement of the shoe forward in the machine direction.
Further, the applicants' own Swedish, still unpublished patent application SE 9901636-2 describes a shoe press for a paper or a paperboard machine. The shoe press comprises a press shoe and a counter roll, which between themselves form an extended nip for passage of a paper or paperboard web and an endless flexible belt therethrough, and at least one press cylinder arranged between a horizontal beam included in the support system of the shoe press and the press shoe, for pressing the press shoe against the counter roll. The cylinder or piston of the press cylinder is mounted in bearings to swivel around an axis in relation to which the axis forming a symmetry axis for the work space of the press cylinder is eccentric. The shoe press further possesses maneuvering members for swivelling of the cylinder or piston of the press cylinder in order to regulate the pressure profile of the shoe press, which maneuvering members comprise an actuating device extending outside the mantle.
In U.S. Pat. No. 5,676,799 a device is described, with an extended press nip for treatment of a fiber web. At the device at least one press surface is formed by a belt subjected to pressure from at least one cylinder-piston unit by way of a hydraulic contact pressure device, consisting of at least one supporting element, whereby a supply of pressurised fluid for pressure pockets of the supporting element is insured by one of the parts of the piston-cylinder unit being permanently connected to a part of the supporting element, whereas the other part of the piston-cylinder unit is kept movable in the machine direction and in the direction of web displacement on one of the elements, and whereby the supporting element is supported by way of a stop attached to the supporting part.
Many of the previously known arrangements for affecting the pressure curve in the machine direction of extended press nips may be experienced as unnecessarily complicated and expensive, and often require an appreciable amount of time to make the adjustments necessary when switching from one pressure curve to another.
Further, there is a frequent risk that the desired adjustment of the pressure curve in the machine direction is disturbed during operation, which may lead to problems concerning operability and quality.